The T cytoplasm undergoes changes during in vitro culture that result in a shift from male sterility to male fertility. The male fertile phenotype exhibits cytoplasmic inheritance and seems to be closely associated with an alteration in the mitochondrial genome (B. G. Gengenbach et al., Theor. Appl. Genet. 59:161-167, 1981). The reversion to male fertility represents a case of cytoplasmic restoration of male sterility because the revertant, male fertile plants should be Rf1 Rf1 rf2 rf2. This suggested to us that tissue culture may be a method of inducing cytoplasmic variants in the normal (N) cytoplasm that restore fertility in plants homozygous for one of the nuclear, recessive male sterile genes. If obtained, such variants would be useful in hybrid seed production by facilitating production of all-male sterile progeny containing normal cytoplasm.
Organogenic callus cultures were initiated from immature embryos heterozygous or homozygous for a known male sterile gene (Table 1). From 3 to 20 months after culture initiation, plants were regenerated and grown to maturity in the glasshouse or field. Male fertile, regenerated plants were self-pollinated. Progeny of these plants were grown in the field and scored for male fertility.
Eleven tissue cultures were established representing 8 male sterile genes. Three cultures were apparently homozygous for the male sterile gene and only produced completely male sterile plants (348). Eight cultures were heterozygous for a male sterile gene and produced male fertile plants (111). Approximately 15% of these plants had high levels of aborted pollen (50-75%), indicating the presence of a chromosomal aberration presumably induced by the tissue culture process. Progeny of most male fertile regenerated plants segregated as expected (3 fertile:1 male sterile). Progeny of two regenerated plants had an excess of male sterile segregants and actually fit a 9:7 (fertile: sterile) ratio. Progeny of one regenerated plant did not segregate for male sterility. These plants were self-pollinated and male sterile segregants were observed in the subsequent generation. Overall, 459 regenerated plants were evaluated for cases of cytoplasmic restoration of male sterility in the normal cytoplasm. No cases of restoration were detected among these plants.
Michael Lee* and R. L. Phillips
*Dept. of Agronomy, lowa State University
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